Update a re-imported event rather than skipping it

Contacts and calendars disagreed on a re-import: a vCard or LDIF entry
whose identity a book already held overwrote the card there (#242, #274),
while an event whose UID a calendar held was counted and thrown away
(#222). The asymmetry was never decided -- it was where each half stopped.

Decided on #279: calendars update too, with two properties held back.
`participants` carries every attendee's accepted/declined and
`recurrenceOverrides` holds every "just this Wednesday" edit made here.
Both are decisions taken after the file was written, and a file that
mentions them at all describes them as they were at export, so writing
either one over would destroy work silently and return no error. A
corrected export now fixes the time, the title and the location, and
leaves who said yes alone. `uid` is held back with them: it is what the
two were matched on, so it is already equal.

The scan returns uid -> id rather than a set of UIDs, since updating
needs something to address, and creates and updates now share one
`maxObjectsInSet` budget the way contacts' `writeCards` does -- 300 new
and 300 changed batched separately would be two calls of 300, neither
over a ceiling of 500 and both refused. Counts become created/updated,
reported as the contacts import reports them.

Still no scheduling messages, on an update as much as on a create. That
is a real cost -- an event a re-import moves is moved here and nowhere
else -- and it is the lesser one: an import is not the place to start
mailing a room full of people who never asked for it.

Driven against the mock end to end: a second file with the same UID
updated the event in place, took the file's title, start and location,
and left an accepted RSVP and a per-occurrence override untouched even
though the file carried participants of its own.

web/src/locales/ru.ts

@@ -1382,8 +1382,7 @@ export const catalog: Catalog = {
     "Your administrator changed {n} settings": { one: "Администратор изменил {n} настройку", few: "Администратор изменил {n} настройки", many: "Администратор изменил {n} настроек", other: "Администратор изменил {n} настройки" },
     "Exported {n} events": { one: "Экспортировано {n} событие", few: "Экспортировано {n} события", many: "Экспортировано {n} событий", other: "Экспортировано {n} события" },
     "Imported {n} events": { one: "Импортировано {n} событие", few: "Импортировано {n} события", many: "Импортировано {n} событий", other: "Импортировано {n} события" },
-    "Already here: {n} events, nothing imported": { one: "Уже есть: {n} событие, ничего не импортировано", few: "Уже есть: {n} события, ничего не импортировано", many: "Уже есть: {n} событий, ничего не импортировано", other: "Уже есть: {n} события, ничего не импортировано" },
-    "{n} were already here": { one: "{n} уже было здесь", few: "{n} уже были здесь", many: "{n} уже были здесь", other: "{n} уже были здесь" },
+    "Updated {n} events, nothing new": { one: "Обновлено {n} событие, новых нет", few: "Обновлено {n} события, новых нет", many: "Обновлено {n} событий, новых нет", other: "Обновлено {n} события, новых нет" },
     /*
      * Three forms, which is the whole reason plural() takes a map rather than
      * (one, other). Intl.PluralRules picks: 1 is `one`, 2-4 are `few`, 5-20